In general terms, a centrifugal compressor for a chiller typically consists of the following components: inlet guide vanes, one or more impellers within a housing surrounded by one or more diffusers with collectors driven by some mechanical shaft means, such as for example, an electric motor. The mechanical shaft means is supported by one or more bearings of the rolling element, journal, or magnetic bearing type which accommodate both radial and axial loads. In variable speed electric chillers, the centrifugal compressor is supplied with electrical power through an adjustable speed motor drive which alters the frequency and/or voltage of the power to the motor to modulate the speed of the compressor.
Rolling element bearings are generally passive devices and, during normal operation, operate without the requirement of active control. The chiller control system does not typically provide active control of the rolling element bearings where, in this context, active implies continual adjustment of some bearing feature. Chiller control systems for centrifugal chillers which use rolling element bearings in the compressor may monitor the bearing temperature, at periodic intervals, as an indication of whether the machine is operating properly. An elevated temperature is used as an indication of a potential mechanical problem with the bearings. If the measured bearing temperature exceeds a predefined set point, the chiller control system may be programmed to stop the machine and alert the user.
In magnetic bearing centrifugal compressors, the compressor rotor is suspended within a magnetic field generated by the magnetic bearings. For definitional purposes, “magnetic bearings” are electromagnetic devices used for suspending a rotating body in a magnetic field without mechanical contact. The bearings can be further classified as active, indicating that some type of active control system is necessary to ensure stable levitation of the rotating body.
Distinct from other compressor types, a magnetic bearing centrifugal compressor uses magnetic bearings as the primary means for supporting the rotor structure. There is a clearance gap between the rotating and stationary components of the bearing that is measurable and controllable. For the magnetic bearings to operate properly, electrical power and proper operation of the magnetic bearing control electronics are required.
In contrast to conventional rolling element bearings, magnetic bearings need to be continuously supplied with electrical energy. If the supply voltage of the magnetic bearings fails, for example as a result of an electrical failure of the power supply system, not only will the motor run down, but the functionality of the magnetic bearings will no longer be provided. In order to avoid damage to the magnetic bearings, the shaft, and/or other components in the event of failure of the supply voltage when the motor is still rotating, so-called emergency operation conditions may be implemented mechanically. However, such emergency operations are only effective for a limited number of failures of the supply voltage of the magnetic bearings without the magnetic bearings, the shaft or other components being damaged. Therefore, applications of magnetic bearings in sites with insufficiently stable electrical power supply systems may be problematic. In such circumstances, the maximum permissible number of emergency operations of the magnetic bearings is quickly reached, with the result that the abovementioned components need to be replaced even after a relatively short period of time. This replacement is generally time-consuming and associated with high costs.
Therefore, backup systems, such as batteries, are often used for safeguarding the magnetic bearings in the event of failure of the electrical supply voltage. Such backup systems maintain the functionality of the magnetic bearing arrangement until the rotating shaft is braked to a sufficient extent such that no damage or wear occurs to the bearings or shaft.
Because the battery/batteries provide the necessary backup power during an electrical utility power outage, it is important that the battery/batteries be operative when such power outage occurs. Therefore, the batteries must be periodically tested to insure that a proper charge is maintained during operation.
Therefore, it would be beneficial to provide an improved battery monitoring system which monitors the condition and state of the battery when the battery is under load and communicates such status to a control unit, thereby allowing a missing, disconnected, weak or dead battery to be detected and appropriate action taken.